An embodiment of the present invention generally relates to X-ray systems utilizing a solid state multiple element X-ray detector for producing an image; and more particularly, to techniques and apparatus for identifying data lines susceptible to line artifacts and for correcting line artifacts.
Solid state X-ray detectors have been proposed that comprise a two dimensional array of 1,000 to 4,000 detector elements in each dimension (x,y). Each detector element comprises a photo detector that detects and stores charge representative of an amount of radiation incident on the detector element. Each detector element further includes a thin film transistor (TFT) connected to the photo diode and operated as a switch to enable and disable read out of the charge stored on the photo diode. Each detector element ultimately produces an electrical signal which corresponds to the brightness of a picture element in the X-ray image projected onto the detector. The signal from each detector element is read out individually and digitized for further image processing, storage and display.
One application of the solid state detector has been for thoracic imaging. During thoracic imaging, it is typical to utilize the entire detector field of view to receive the X-ray beam. The detector field of view is entirely covered by the patient during thoracic applications. Because the X-ray beam is transmitted through the body of the patient before impinging anywhere upon the detector, typically no region of the detector receives a high level of radiation.
More recently, the solid state X-ray detector has been proposed for use in general radiology applications, such as imaging an arm, knee, hand, or any other part or parts of the body that would not utilize the entire field of view. Since a smaller part of the body is being imaged, the patient does not entirely cover the detector field of view. Hence, some regions of the detector may be exposed to greater amounts of radiation than other areas. For example, if an image of a foot is taken, the foot may cover only a portion of the detector. Thus some regions of the detector may receive a relatively high level of radiation, while other regions may receive a relatively low level of radiation. In this instance, a region of the detector may be exposed to a level of radiation great enough such that the signal level is sufficient to cause the TFT to begin to conduct, or xe2x80x9cleakxe2x80x9d, even while maintained in the OFF state. This signal level is referred to as the TFT leakage threshold. The TFT leakage threshold may not be the same for all configurations of detector elements. For example, the TFT may begin to conduct when the diode is only at one half saturation if the detector element includes a TFT and a storage capacitor, but does not include a light shield. It is also possible that the TFT may begin to conduct only if the signal level is, for example, at least five times the level necessary to saturate the diode if the detector element includes a low capacitance diode.
When a TFT begins to conduct while in the OFF state, charge on the data line may occur. Typically, detector elements are read out in rows or columns. For example, when a column of detector elements is read out, the charges stored in the detector elements within the present column are sequentially read row by row. Detector elements not presently being read are maintained OFF in order that a charge read out on a particular line may be correlated to one detector element.
However, when a TFT leaks charge while OFF, it adds charge to the output line for a column thereby causing an increased charge to be correlated to a different detector element. If the detector elements are then read in a manner such that the region that received a low level of radiation (i.e. a level of radiation below the TFT leakage threshold) is read out before the region that received a high level of radiation (i.e. a level of radiation equal to or above the TFT leakage threshold), then the TFT may begin to conduct and leak charge onto the output line even while the region that received the high level of radiation is not being read. The leakage charge adds a bias to the read out of detector elements in regions that received a low level of radiation and appears (if not corrected) as line artifacts. Hence, line artifacts may occur in the region that received a low level of radiation due to differences between leakage signals on adjacent data lines.
Methods have been proposed for identifying and correcting image artifacts that may be caused by faulty detector elements, or other anomalies present in the X-ray detector, which appear in the acquired image as bad pixels. The bad pixels are identified during the evaluation of calibration images. These calibration images may be created by exposing the detector to background radiation or to a level of radiation uniform across the detector. When bad pixels are identified in this manner, they are added to the detector""s bad pixel map and are thereafter corrected in all applications and procedures. An additional method exists to identify and correct bad pixels as data is acquired. This method compares each pixel to a predetermined threshold and corrects each pixel that meets the criteria.
However, neither of these methods, identifying bad pixels by evaluating calibration images or comparing the pixel data as it is acquired to a predetermined value, will identify pixels that cause line artifacts when a detector is exposed to a non-uniform level of radiation and TFT leakage occurs. Thus, if an artifact is created only under certain circumstances, conventional methods may not identify susceptible pixels. As a consequence, it is desirable to be able to identify which detector elements may cause line artifacts when TFT leakage occurs. It is further desirable to correct the line artifact only after it has been determined that the line artifact exists, and also to correct only the pixels exhibiting the line artifact.
In accordance with at least one embodiment, a method is provided to identify detector elements, formed in rows and columns defining lines in a solid state X-ray detector, susceptible to causing line artifacts due to thin film transistor (TFT) leakage. A portion of the X-ray detector is covered by a radiation occluding material and the detector is exposed to a level of radiation sufficient to cause the TFT in a detector element in the exposed portion of the detector to conduct. An image is acquired representative of the amount of radiation detected. The detector elements are analyzed to determine whether line artifacts are present. In accordance with an alternative embodiment, during the analyzing step the acquired image is analyzed to determine whether line artifacts are present. Any data lines in the detector found to exhibit line artifacts are stored in the image processor. In accordance with an alternative embodiment, before analysis the resultant image is filtered to remove low frequency shading, and in another alternative embodiment, the covered portion of the detector is filtered to remove low frequency shading.
In accordance with at least one alternative embodiment, during the analyzing step a value is calculated for each line of the X-ray detector representing the radiation detected by detector elements in the covered portion of the line. In one embodiment, the line corresponds to a column of the detector, while in another embodiment the line corresponds to a row of the detector. The data values representative of at least the charge on the detector elements for each line along the covered portion of the line are summed and analyzed with respect to a predetermined threshold. In accordance with at least one embodiment, at least one data value includes a charge component from a detector element in the covered portion and a leakage component from a detector element in the exposed portion.
In accordance with at least one embodiment, a method is provided to correct line artifacts in a solid state X-ray detector caused by charge leakage of a TFT, a component of each detector element. The X-ray detector is exposed to radiation and an image is acquired representative of an amount of radiation detected by the detector elements. The data lines that were previously found to exhibit line artifacts are analyzed with respect to a predetermined threshold. The level of radiation exposure from the X-ray generator is then calculated with respect to a predetermined threshold and pixel correction is performed if it is required.
In accordance with at least one alternative embodiment, during the analyzing step it is determined for each line independently whether any data value exceeds or does not exceed the predetermined threshold. In one embodiment, the line corresponds to a column of the detector, while in another embodiment the line corresponds to a row of the detector. The data values are then analyzed to determine whether one or more data values first detected by the detector elements did not exceed the predetermined threshold.